The present invention relates generally to the field of DC drive systems and, in certain embodiments, DC drive systems for delivering current to drive a motor or for other DC power module applications, such as a DC bus supply for an AC inverter drive or a DC power module, as well as for various other applications, such as electroplating.
DC drive systems generally include a drive regulator coupled to a power module. The power module may be configured as a plurality of switching devices. The drive regulator generates gate firing timing pulses based on a condition detectable by detection circuitry. For example, the drive regulator may generate gate firing timing pulses based on the desired control of motor current, torque, speed or shaft position and the zero crossings of an AC line input. The AC line zero crossings are detected to establish the exact firing time of the power devices (e.g. SCRs) relative to the AC line. The gate firing timing pulses drive the switching devices in the power module, thereby generating a direct current output to drive a load, for example a motor.
As high power applications, such as high horsepower motors and large steel rolling mills, have become increasing popular, the demand for higher output DC drive systems has also increased. Along with the increase in horsepower is the demand to control the amount of harmonics generated in the AC power lines along with providing “smooth power” to the motor. The voltage to each of the power modules is phase shifted by means of the power transformers that supply them with power. The phase shift of the voltage between each of the input voltages depends on the topology of the system. In the topology known in the industry as an S12/S12R, the phase shift would be 30 degrees, for an S18/S18R, 20 degrees, whereas an S24/S24R requires 15 degrees, and so forth. One current solution of these DC drive systems is to connect multiple power modules in parallel, with each module being governed by its own drive regulator which regulates an independent current loop and generates a gate firing pattern for each power module. The current between each power module is phase shifted accordingly depending on the number of power modules and the configuration of switching devices in each power module. The output currents of each power module are then combined using summing circuitry (typically an inductor), prior to being delivered to the motor or other DC load.
Although the use of multiple independent drive regulators for controlling multiple power modules in DC drive systems functions adequately, the technique is not without drawbacks. The independent gate firing patterns from each regulator may result in difficulty tuning the drive system due to current instability. When the drive regulators become unstable and/or out of phase with one another, protection circuitry (i.e., circuit breakers and fuses) may engage and shut down the system. Such protective measures, while necessary, are burdensome and hinder production efficiency.
In order to avoid drawbacks of the prior art, there is a need, therefore, for an improved DC motor drive system having more stable tuning features to enable higher bandwidth for increased output and improved drive performance.